Not Applicable
1. Field of the Invention
The present invention relates generally to the modification and control of the temperature of a selected body organ. More particularly, the invention relates to a method and intravascular apparatus for controlling organ temperature.
2. Background Information
Organs in the human body, such as the brain, kidney and heart, are maintained at a constant temperature of approximately 37xc2x0 C. Hypothermia can be clinically defined as a core body temperature of 35xc2x0 C. or less. Hypothermia is sometimes characterized further according to its severity. A body core temperature in the range of 33xc2x0 C. to 35xc2x0 C. is described as mild hypothermia. A body temperature of 28xc2x0 C. to 32xc2x0 C. is described as moderate hypothermia. A body core temperature in the range of 24xc2x0 C. to 28xc2x0 C. is described as severe hypothermia.
Hypothermia is uniquely effective in reducing brain injury caused by a variety of neurological insults and may eventually play an important role in emergency brain resuscitation. Experimental evidence has demonstrated that cerebral cooling improves outcome after global ischemia, focal ischemia, or traumatic brain injury. For this reason, hypothermia may be induced in order to reduce the effect of certain bodily injuries to the brain as well as other organs.
Cerebral hypothermia has traditionally been accomplished through whole body cooling to create a condition of total body hypothermia in the range of 20xc2x0 C. to 30xc2x0 C. However, the use of total body hypothermia risks certain deleterious systematic vascular effects. For example, total body hypothermia may cause severe derangement of the cardiovascular system, including low cardiac output, elevated systematic resistance, and ventricular fibrillation. Other side effects include renal failure, disseminated intravascular coagulation, and electrolyte disturbances. In addition to the undesirable side effects, total body hypothermia is difficult to administer.
Catheters have been developed which are inserted into the bloodstream of the patient in order to induce total body hypothermia. For example, U.S. Pat. No. 3,425,419 to Dato describes a method and apparatus of lowering and raising the temperature of the human body. Dato induces moderate hypothermia in a patient using a metallic catheter. The metallic catheter has an inner passageway through which a fluid, such as water, can be circulated. The catheter is inserted through the femoral vein and then through the inferior vena cava as far as the right atrium and the superior vena cava. The Dato catheter has an elongated cylindrical shape and is constructed from stainless steel. By way of example, Dato suggests the use of a catheter approximately 70 cm in length and approximately 6 mm in diameter. However, use of the Dato device implicates the negative effects of total body hypothermia described above.
Due to the problems associated with total body hypothermia, attempts have been made to provide more selective cooling. For example, cooling helmets or head gear have been used in an attempt to cool only the head rather than the patient""s entire body. However, such methods rely on conductive heat transfer through the skull and into the brain. One drawback of using conductive heat transfer is that the process of reducing the temperature of the brain is prolonged. Also, it is difficult to precisely control the temperature of the brain when using conduction due to the temperature gradient that must be established externally in order to sufficiently lower the internal temperature. In addition, when using conduction to cool the brain, the face of the patient is also subjected to severe hypothermia, increasing discomfort and the likelihood of negative side effects. It is known that profound cooling of the face can cause similar cardiovascular side effects as total body cooling. From a practical standpoint, such devices are cumbersome and may make continued treatment of the patient difficult or impossible.
Selected organ hypothermia has been accomplished using extracorporeal perfusion, as detailed by Arthur E. Schwartz, M.D. et al., in Isolated Cerebral Hypothermia by Single Carotid Artery Perfusion of Extracorporeally Cooled Blood in Baboons, which appeared in Vol. 39, No. 3, NEUROSURGERY 577 (September, 1996). In this study, blood was continually withdrawn from baboons through the femoral artery. The blood was cooled by a water bath and then infused through a common carotid artery with its external branches occluded. Using this method, normal heart rhythm, systemic arterial blood pressure and arterial blood gas values were maintained during the hypothermia. This study showed that the brain could be selectively cooled to temperatures of 20xc2x0 C. without reducing the temperature of the entire body. However, external circulation of blood is not a practical approach for treating humans because the risk of infection, need for anticoagulation, and risk of bleeding is too great. Further, this method requires cannulation of two vessels making it more cumbersome to perform particularly in emergency settings. Even more, percutaneous cannulation of the carotid artery is difficult and potentially fatal due to the associated arterial wall trauma. Finally, this method would be ineffective to cool other organs, such as the kidneys, because the feeding arteries cannot be directly cannulated percutaneously.
Selective organ hypothermia has also been attempted by perfusion of a cold solution such as saline or perflourocarbons. This process is commonly used to protect the heart during heart surgery and is referred to as cardioplegia. Perfusion of a cold solution has a number of drawbacks, including a limited time of administration due to excessive volume accumulation, cost, and inconvenience of maintaining the perfusate and lack of effectiveness due to the temperature dilution from the blood. Temperature dilution by the blood is a particular problem in high blood flow organs such as the brain.
Therefore, a practical method and apparatus which modifies and controls the temperature of a selected organ satisfies a long-felt need.
The apparatus of the present invention can, by way of example only, include a heat transfer element which comprises first and second elongated, articulated segments, each segment having a turbulence-inducing exterior surface. A flexible joint can connect the first and second elongated segments. An inner coaxial lumen may be disposed within the first and second elongated segments and is capable of transporting a pressurized working fluid to a distal end of the first elongated segment. In addition, the first and second elongated segments may have a turbulence-inducing interior surface for inducing turbulence within the pressurized working fluid. The turbulence-inducing exterior surface may be adapted to induce turbulence within a free stream of blood flow when placed within an artery. The turbulence-inducing exterior surface may be adapted to induce a turbulence intensity greater than 0.05 within a free stream blood flow. In one embodiment, the flexible joint comprises a bellows section which also allows for axial compression of the heat transfer element.
In one embodiment, the turbulence-inducing exterior surfaces of the heat transfer element comprise one or more helical ridges configured to have a depth which is greater than a thickness of a boundary layer of blood which develops within an arterial blood flow. Adjacent segments of the heat transfer element can be oppositely spiraled to increase turbulence. For instance, the first elongated heat transfer segment may comprise one or more helical ridges having a counter-clockwise twist, while the second elongated heat transfer segment comprises one or more helical ridges having a clockwise twist. Alternatively, of course, the first elongated heat transfer segment may comprise one or more clockwise helical ridges, and the second elongated heat transfer segment may comprise one or more counter-clockwise helical ridges. The first and second elongated, articulated segments may be formed from highly conductive materials.
In another embodiment, the turbulence-inducing exterior surface of the heat transfer element is adapted to induce turbulence throughout the duration of each pulse of a pulsatile blood flow when placed within an artery. In still another embodiment, the turbulence-inducing exterior surface of the heat transfer element is adapted to induce turbulence during at least 20% of the period of each cardiac cycle when placed within an artery.
The heat transfer device may also have a coaxial supply catheter with an inner catheter lumen coupled to the inner coaxial lumen within the first and second elongated heat transfer segments. A working fluid supply configured to dispense the pressurized working fluid may be coupled to the inner catheter lumen. The working fluid supply may be configured to produce the pressurized working fluid at a temperature of about 0xc2x0 C. and at a pressure below about 5 atmospheres of pressure.
In yet another alternative embodiment, the heat transfer device may have three or more elongated, articulated, heat transfer segments having a turbulence-inducing exterior surface, with additional flexible joints connecting the additional elongated heat transfer segments. In one such embodiment, by way of example, the first and third elongated heat transfer segments may comprise clockwise helical ridges, and the second elongated heat transfer segment may comprise one or more counter-clockwise helical ridges. Alternatively, of course, the first and third elongated heat transfer segments may comprise counter-clockwise helical ridges, and the second elongated heat transfer segment may comprise one or more clockwise helical ridges.
The turbulence-inducing exterior surface of the heat transfer element may optionally include a surface coating or treatment to inhibit clot formation. One variation of the heat transfer element comprises a stent coupled to a distal end of the first elongated heat transfer segment.
The present invention also envisions a method of treating the brain which comprises the steps of inserting a flexible, conductive heat transfer element into a carotid artery from a distal location, and circulating a working fluid through the flexible, conductive heat transfer element in order to selectively modify the temperature of the brain without significantly modifying the temperature of the entire body. The flexible, conductive heat transfer element preferably absorbs more than about 25, 50 or 75 Watts of heat.
The method may also comprise the step of inducing turbulence within the free stream blood flow within the carotid artery. In one embodiment, the method includes the step of inducing blood turbulence with a turbulence intensity greater than about 0.05 within the carotid artery. In another embodiment, the method includes the step of inducing blood turbulence throughout the duration of the period of the cardiac cycle within the carotid artery. In yet another embodiment, the method comprises the step of inducing blood turbulence throughout the period of the cardiac cycle within the carotid artery or during greater than about 20% of the period of the cardiac cycle within the carotid artery. The step of circulating may comprise the step of inducing turbulent flow of the working fluid through the flexible, conductive heat transfer element. The pressure of the working fluid may be maintained below about 5 atmospheres of pressure.
The present invention also envisions a method for selectively cooling an organ in the body of a patient which comprises the steps of introducing a catheter, with a heat transfer element, into a blood vessel supplying the organ, the catheter having a diameter of about 4 mm or less, inducing free stream turbulence in blood flowing over the heat transfer element, and cooling the heat transfer element to remove heat from the blood to cool the organ without substantially cooling the entire body. In one embodiment, the cooling step removes at least about 75 Watts of heat from the blood. In another embodiment, the cooling step removes at least about 100 Watts of heat from the blood. The organ being cooled may be the human brain.
The step of inducing free stream turbulence may induce a turbulence intensity greater than about 0.05 within the blood vessel. The step of inducing free stream turbulence may induce turbulence throughout the duration of each pulse of blood flow. The step of inducing free stream turbulence may induce turbulence for at least about 20% of the duration of each pulse of blood flow.
In one embodiment, the catheter has a flexible metal tip and the cooling step occurs at the tip. The tip may have turbulence-inducing elongated heat transfer segments separated by bellows sections. The turbulence-inducing segments may comprise helical ridges which are configured to have a depth which is greater than a thickness of a boundary layer of blood which develops within the blood vessel. In another embodiment, the catheter has a tip at which the cooling step occurs and the tip has turbulence-inducing elongated heat transfer segments that alternately spiral bias the surrounding blood flow in clockwise and counterclockwise directions.
The cooling step may comprise the step of circulating a working fluid in through an inner lumen in the catheter and out through an outer, coaxial lumen. In one embodiment, the working fluid remains a liquid throughout the cycle. The working fluid may be aqueous.
The present invention also envisions a cooling catheter comprising a catheter shaft having first and second lumens therein. The cooling catheter also comprises a cooling tip adapted to transfer heat to or from a working fluid circulated in through the first lumen and out through the second lumen, and turbulence-inducing structures on the cooling tip capable of inducing free stream turbulence when the tip is inserted into a blood vessel. The turbulence-inducing structures may induce a turbulence intensity of at least about 0.05. The cooling tip may be adapted to induce turbulence within the working fluid. The catheter is capable of removing at least about 25 Watts of heat from an organ when inserted into a vessel supplying that organ, while cooling the tip with a working fluid that remains a liquid in the catheter. Alternatively, the catheter is capable of removing at least about 50 or 75 Watts of heat from an organ when inserted into a vessel supplying that organ, while cooling the tip with an aqueous working fluid. In one embodiment, in use, the tip has a diameter of about 4 mm or less. Optionally, the turbulence-inducing surfaces on the heat transfer segments comprise helical ridges which have a depth sufficient to disrupt the free stream blood flow in the blood vessel. Alternatively, the turbulence-inducing surfaces may comprise staggered protrusions from the outer surfaces of the heat transfer segments, which have a height sufficient to disrupt the free stream flow of blood within the blood vessel.
In another embodiment, a cooling catheter may comprise a catheter shaft having first and second lumens therein, a cooling tip adapted to transfer heat to or from a working fluid circulated in through the first lumen and out through the second lumen, and turbulence-inducing structures on the cooling tip capable of inducing turbulence when the tip is inserted into a blood vessel. Alternatively, a cooling catheter may comprise a catheter shaft having first and second lumens therein, a cooling tip adapted to transfer heat to or from a working fluid circulated in through the first lumen and out through the second lumen, and structures on the cooling tip capable of inducing free stream turbulence when the tip is inserted into a blood vessel. In another embodiment, a cooling catheter may comprise a catheter shaft having first and second lumens therein, a cooling tip adapted to transfer heat to or from a working fluid circulated in through the first lumen and out through the second lumen, and turbulence-inducing structures on the cooling tip capable of inducing turbulence with an intensity greater than about 0.05 when the tip is inserted into a blood vessel.